Melt half the butter in a heavy-based saucepan over a low heat. Add the onion and cook for 10 minutes until softened. Add the rice and cook, stirring, for 1 minute. Pour in the wine and cook, stirring, until absorbed.
Add 2 ladlefuls of the simmering stock and the saffron. Slowly simmer, stirring constantly, until the stock has been absorbed and the rice parts when a wooden spoon is run through it. Add another ladleful of stock and continue to cook, stirring and adding the stock in ladlefuls, for 18–20 minutes until the rice is creamy and almost tender to the bite.
Remove from the heat and stir in the Parmesan and remaining butter. Stir vigorously for 15 seconds. Cover with a tight-fitting lid and leave to stand for 1 minute. Serve immediately with extra Parmesan on the side.